Chemical sensors in which the electrical resistance of a sensitive layer, usually comprised of metal oxides, can be evaluated with the aid of an evaluating structure formed by the electrodes, are known in a wide variety of configurations especially for use as gas or moisture sensors. As sensitive layers for gas detection, as a rule, porous ceramic layers, for example SnO2 or WO3 are used because their electrical surface conductivity varies with adsorption of gases. The porous ceramic layers can be made selectively sensitive to certain gases by the use of doping agents.
The resistivity of such ceramics is very high. As a result the measurement resistance is also large. The evaluation structure, comprised usually of an interdigital structure (IDT; “Interdigitated Transducers”), may have two coplanar electrodes with fingers which interdigitate in their plane. This corresponds to parallel networks with different polarities between the two fingers across resistances which thereby have a reduced internal sensor resistance or increased sensitivity of the sensor.
Generally apart from the electrodes and the heating resistance, a temperature measurement resistance can be provided for the sensor, whereby all of the metallization elements can be structured in a single metallization plane and, for example, can be composed of platinum. To reduce the aging effect, especially of the temperature measurement resistance, in many cases a passivation layer is applied to the metallization plane, typically of silicon oxide. The passivation layer can be structured by contact holes through which contact is enabled between the electrodes and the sensitive layer applied to the passivation layer.
While the metallization and sensitive layer structures are conventionally applied to an aluminum oxide substrate, micromechanical membrane sensors are also known which use as a basis, a silicon substrate. By thermal decoupling of the sensor structure on the membrane from the substrate, the sensor can have a reduced power consumption.
A microstructured silicon membrane sensor with a sensitive layer applied to an SiO2, Si3N4 membrane is known for example from DE 197 10 358 (U.S. equivalent U.S. Pat. No. 6,787,047). With this known sensor, in contrast to the sensor of the invention, only the heater and temperature measurement structures are passivated with a silicon oxide layer which is applied to an interdigitated three dimensional electrode structure and in which the sensor layer is applied by a silk screen technique to fill in the electrode structure.
In general with silk-screen formation of the sensitive ceramic layer, the layer is in the form of a thick film paste which must then be sintered. The result can be mechanical stresses which may arise or remain, especially in the form of interfacial or boundary layer stresses which can result in separation of the layer material and the generation of particles. The effect of for example SnO2 or WO3 on micromechanical processes is unclear so that in general a contamination danger arises.
While the porosity of the metal-oxide ceramic which results from the sintering is desirable, on the one hand since the high sensitivity depends upon the high surface volume ratio, on the other hand the porosity has a negative effect on the mechanical stability of the layer. The most important requirement for stability is that the ceramic layer and the electrodes remain adherent to their support surfaces over the life of the sensor. Furthermore, the electrical contact between the ceramic and the electrodes should not degenerate.
Up to now, the ceramic layers have been applied directly to the passivation layer. It has been found however that the adhesion of the ceramic in many instances is insufficient.